[PATCH 0/7] efi: Various tidy-ups and drop the default

[Mark Kettenis]

> From: Simon Glass <sjg at chromium.org>
> Date: Fri, 2 Jul 2021 13:05:20 -0600
> Cc: Tom Rini <trini at konsulko.com>, U-Boot Mailing List <u-boot at lists.denx.de>,
>         Pali Rohár <pali at kernel.org>,
>         Heinrich Schuchardt <xypron.glpk at gmx.de>,
>         Ilias Apalodimas <ilias.apalodimas at linaro.org>,
>         Alex Graf <agraf at csgraf.de>,
>         Masahiro Yamada <yamada.masahiro at socionext.com>
> Content-Type: text/plain; charset="UTF-8"
> 
> Hi Mark,
> 
> On Mon, 28 Jun 2021 at 11:49, Mark Kettenis <mark.kettenis at xs4all.nl> wrote:
> >
> > > From: Simon Glass <sjg at chromium.org>
> > > Date: Mon, 28 Jun 2021 08:18:26 -0600
> > >
> > > Hi Tom, Mark,
> > >
> > > On Mon, 28 Jun 2021 at 07:37, Tom Rini <trini at konsulko.com> wrote:
> > > >
> > > > On Mon, Jun 28, 2021 at 10:38:50AM +0200, Mark Kettenis wrote:
> > > > > > From: Simon Glass <sjg at chromium.org>
> > > > > > Date: Sun, 27 Jun 2021 19:48:34 -0600
> > > > > >
> > > > > > It has come to light that EFI_LOADER adds an extraordinary amount of
> > > > > > code to U-Boot. For example, with nokia_rx51 the size delta is about
> > > > > > 90KB. About 170 boards explicitly disable the option, but is is clear
> > > > > > that many more could, thus saving image size and boot time.
> > > > >
> > > > > EFI_LOADER used to be a lot smaller.  It is great to see that over the
> > > > > years UEFI support has become more complete, but a lot of that new
> > > > > code implements features that are not at all essential for just
> > > > > booting an OS from storage.  If that growth leads to the suggestion to
> > > > > disable EFI_LOADER completely by default, we're putting the cart
> > > > > before the horse.
> > > >
> > > > Well, I see I forgot to prefix my patch with RFC, but I hadn't found
> > > > EFI_LOADER being used in the wild on armv7, but wasn't sure about the
> > > > BSD families.  I did see that Debian doesn't use it, and that Armbian
> > > > doesn't even use it on aarch64.
> > > >
> > > > > > The current situation is affecting U-Boot's image as a svelt bootloader.
> > > > >
> > > > > Really?  I know UEFI has a bad reputation in the Open Source world,
> > > > > and some of its Microsoft-isms are really annoying (yay UCS-2).  But
> > > > > it works, it provides a standardized approach across several platforms
> > > > > (ARMv7, AMRv8, RISC-V) and the industry seems to like it.  Personally
> > > > > I'd wish the industry had standardized on Open Firmware instead, but
> > > > > that ship sailed a long time ago...
> > > > >
> > > > > I find it hard to imagine that 90k is a serious amount of storage for
> > > > > something that is going to include a multi-MB Linux kernel.  This
> > > > > isn't code that lives in SPL or TPL where severe size restrictions
> > > > > apply.
> > > >
> > > > In one of those cases where I need to pop back in to the other (Nokia
> > > > N900 specific) thread and see if the big size reduction really was just
> > > > disabling EFI_LOADER, it's perhaps just one of those "fun" things about
> > > > Kconfig and anything other than "make oldconfig" for spotting new config
> > > > options that default to enabled.
> > >
> > > Yes it will be interesting to see what you find there. My results on
> > > nokia_rx51 were something like this:
> > >
> > > default
> > >         arm: (for 1/1 boards) all +129370.0 bss +1136.0 data +7399.0
> > > rodata +10989.0 text +109846.0
> > >
> > > without ebbr
> > >        arm: (for 1/1 boards) all +38460.0 bss +1040.0 data +2375.0
> > > rodata +5333.0 text +29712.0
> > >
> > > with various other things:
> > > CONFIG_OF_LIBFDT_ASSUME_MASK=7
> > > # CONFIG_OF_TRANSLATE is not set
> > > # CONFIG_SIMPLE_BUS is not set
> > > # CONFIG_TI_SYSC is not set
> > > # CONFIG_CMD_FDT is not set
> > >
> > >        arm: (for 1/1 boards) all +19170.0 bss -16.0 data +360.0 rodata
> > > +3274.0 text +15552.0
> > >
> > > (Mark, in the same email:)
> > > > > FIT simply isn't fit for purpose (pun intended).  It only really works
> > > > > for booting Linux, and forces people to combine u-boot, kernel,
> > > > > initial ramdisk and other firmware components into a single image.
> > > > > That is really undesirable as:
> > > > > - This makes it sigificantly harder to update individual components of
> > > > >   such an image.  Making it hard to update a kernel is obviously a
> > > > >   serious security risk.
> > > > > - This makes it impossible to build an OS install image that works om
> > > > >   multiple boards/SoCs.
> > >
> > >
> > > I would really like to understand this better. The whole thing is a
> > > complete mystery to me.
> > >
> > > Firstly I have sometimes fiddled with booting other OSes using FIT. It
> > > seemed OK. I can't see why it only works with Linux.
> >
> > Well, you could of course rework the boot flow of other OSes such that
> > booting them includes some sort of FIT if you really wanted to.  I But
> > an OS like OpenBSD comes with its own bootloader that is essential in
> > the boot flow.  On OpenBSD armv7/arm64/riscv64 it adds some essential
> > properties to the device tree.  Besides, the kernel itself relies on a
> > valid EFI memory map.
> 
> (thanks for taking the time to reply with so much detail)
> 
> That's news to me; when did that happen? Anyway, I doubt that adds a
> lot of code.

Shortly after EFI_LOADER support was added to U-Boot and there was a
clear consensus that EFI_LOADER support was going to be enabled by
default on armv7 and armv8 targets.  My initial commit of the EFI
bootloader for armv7 is from May 2016.

> > > Secondly, I don't expect that U-Boot itself would be in the FIT.
> >
> > So the FIT would only contain the OS kernel and other OS components?
> > What about the FIT that is used on some arm64 platforms to combine
> > U-Boot and TF-A?  I guess you can have multiple FITs...
> >
> > > Thirdly, do you really want the kernel and initrd to be separate? At
> > > least in the systems I have used, they are built together, even having
> > > the same name, e.g.:
> > >
> > > initrd.img-5.10.40-1rodete1-amd64
> > > System.map-5.10.40-1rodete1-amd64
> > > vmlinuz-5.10.28-1rodete2-amd64
> >
> > I don't really use Linux on these platforms.  But I'd expect the
> > normal package management tools of my Linux distribution to build
> > those as necessary and place them in the root file system where the
> > bootloader picks them up.  And as a distro developer, I'd like to have
> > the same approach work on all Linux systems, regardless the specific
> > firmware they're running (EDK2, U-Boot or something completely
> > different).  Ideally that wouldn't even depend on the architecture.
> >
> > Now Armbian takes a different approach, and does treat most systems
> > they provide as special snowflakes, providing flash images for each
> > board.  But that doesn't scale and makes for a fairly crappy user
> > experience.  They don't always support booting a mainline kernel.  And
> > updating the kernel often requires installing special packages.
> 
> I don't think it is a requirement that things have to be special
> snowflakes. There are a few common boot flows to support and we can
> put that support in U-Boot and in userspace, without needing to make
> everything special.
> 
> >
> > > Finally, for the firmware components, do you mean system firmware? If
> > > so, I would expect it to be more convenient to distribute updates to
> > > that separately, although I suppose they could be combined with the
> > > kernel if the combinatorial explosion can be contained. What is the
> > > problem, exactly? (If you mean peripheral firmware, I would expect
> > > fwupd to handle that.)
> >
> > I guess I mean system firmware.  Essentially everything that runs on
> > the system before my OS bootloader runs.  So for me, U-Boot is part of
> > the system firmware even if it sometimes happens to live on removable
> > media.
> >
> > > What exactly is impossible? Can you please be more specific?
> >
> > So let me explain what we want for OpenBSD.  We really want a uniform
> > user experience across platforms, and don't want to maintain different
> > codepaths for special snowflake platforms that might exist for a
> > specific architecture.
> >
> > Installing OpenBSD on a machine should be as simple as dd'ing the
> > installer to some boot media, plugging it in and powering the machine
> > on.  Now this is somewhat tricky to achieve on some hardware targetted
> > by U-Boot as they don't come with usable system firmware on the board
> > itself.  But on those boards you can mostly get away with having
> > U-Boot on uSD or eMMC and the OS installer on USB.
> >
> > Updating the OpenBSD kernel should be as simple as copying the kernel
> > to /bsd.  Since the root filesystem uses the UFS/FFS filesystem, this
> > means that whatever we use as a bootloader must be able to read from
> > that filesystem.  To make sure the kernel is properly seeded with
> > entropy, the OpenBSD bootloader has some additional tricks up its
> > sleeve.  It will replace a special segment in the kernel with random
> > data before handing control to the kernel.  On platforms that support
> > it, it will try to use a firmware-provided RNG to do this (and EFI
> > supports this) but also mix in some random data from a file on the
> > UFS/FFS filesystem.  It will actually mark that file as "used" after
> > reading it to throw a warning when the file is reused when the machine
> > is rebooted (it should have been filled with fresh new entropy).  So
> > you really need to use the OpenBSD bootloader instead loading an
> > OpenBSD kernel directly from system firmware.
> >
> > Updating the OpenBSD bootloader should be as simple as running
> > installboot(8) from within the OS.
> >
> > This all works on pretty much any architecture that OpenBSD supports.
> > And right now, thanks to EFI_LOADER support in U-Boot, we have a
> > nearly uniform boot flow on amd64, arm64, armv7 and riscv64.
> 
> OK I see. Really it sounds like you have a pre-kernel loader. The
> functionality (some fresh random data) could just as easily be
> provided by U-Boot directly, with vastly less code and complexity. But
> I do understand that trying to design across a whole system is a pain,
> and it is attractive to try to use what hooks exist already to do what
> you want.

What do you mean with vastly less code and complexity.  At this point
70% of the OpenBSD bootloader code is shared between most of the
architectures OpenBSD runs on (alpha, amd64, arm64, armv7, hppa, i386,
powerpc, riscv64, sh, sparc64).  And on platforms that support UEFI
(amd64, arm64, armv7 and riscv64) this is closer to 95% shared code.

Having OS-dependent code in U-Boot doesn't work.  FreeBSD tried that
with the U-Boot API.  It was never enabled by default, and got broken
all the time.

> How do you do verified boot?

We don't.  I don't think it makes sense for an Open Source OS where
people like to tinker with things.  And it gets in the way of some of
our own security features.  OpenBSD relinks the kernel upon every boot
as a defence against attacks that depend on the kernel address space
layout.  Doing that in an environment where all code needs to be
signed is a big challenge.

There is a comany that has products based on OpenBSD.  They do
verified boot.  I'm not really familliar with the details, but
presumably they turned off address space randomization and I believe
it is based on EFI_LOADER support.  Patrick Wildt contributed patches
to implement this to U-Boot before the Linaro folks did their
full-blown UEFI-based implementation.

> > > FIT is just a container. It seems to have been rejected by the EFI
> > > crew at some point. Perhaps I just need to try to use it with one of
> > > the distros out there, to actually understand what is going on here.
> > > But any help is appreciated.
> >
> > It just doesn't make sense for us to use a container just because the
> > system firmware (U-Boot) insists on it.  The kernel lives in the root
> > UFS/FFS filesystem and the bootloader lives on an MS-DOS filesystem on
> > the root disk.
> 
> It isn't so much that U-Boot insists on it. It is quite happy to load
> a kernel and initrd separately. But it does make verification harder
> and I assume it makes upgrades harder since there are multiple files
> to install. FIT is just such a nice format for packaging kernels and
> related things. It sounds like you could use FIT and everything you
> said above would still be true.

It would mean a kernel update would need to update the FIT container.
But only on boards that use U-Boot to boot, since nobody else supports
it.  That isn't helpful to us.

Cheers,

Mark